1. Field of the Invention
The present invention relates to an aluminum alloy for rapidly cooled welding, such as laser welding, i.e., welding construction material such as rolling materials and extruded materials, and aluminum thin sheet materials for automobiles. The present invention particularly relates to an aluminum alloy for rapidly cooled welding, and to a welding method therefor, in which welded portion is not softened and welding joint strength is high.
2. Related Art
Aluminum alloys are lighter than steel materials and are effective for lightening structures. Therefore, aluminum alloys have been widely used as materials for automobiles, railroad vehicles, and ships. It is preferable that welding joint efficiency, which is a ratio of welding joint strength to parent material strength, be not less than 1.0, since structural members are assembled by welding. In the aluminum alloys, a heat treatment type alloy, such as a Japanese Industrial Standard (JIS) A6000 type alloy, has high strength. However, when the heat treatment type alloy is welded, precipitates are solution treated by weld heat input, whereby the welded portion is softened and a subsequent heat treatment is required. A JIS A5000 type alloy is a non-heat treatment type alloy and is strengthened by Mg. The strength of a JIS A5000 type alloy is not decreased compared to that of a JIS A6000 type aluminum alloy by welding. However, a JIS A5000 type alloy is annealed by weld heat input and is decreased in strength due to coarsening of the grains, whereby the welding joint efficiency is mostly less than 1.0. Therefore, a technique in which Mg content is increased, and a technique in which Sc is added to increase the strength as shown in Japanese Patent Unexamined (KOKAI) Publication No.10-237577, has been considered.
However, when strength is increased by adding Sc, the production cost is high because Sc is expensive. Moreover, although increasing Mg content is effective for strengthening the welded portion, hot workability is decreased when Mg content is larger than a predetermined amount, whereby the Mg content has a limit. (See Japanese Patent Unexamined (KOKAI) Publication No. 10-237577, p.1)